Potentials of monitoring rock fall hazards by GPR: considering as example the results of Salzburg

A part of the IUGS-UNESCO research project "Landslide Hazard Assessment and Cultural Heritage" (IGCP-Project No. 425) includes testing the application of geophysical methods for assessing hazard risks to people and cultural assets. In this work, Austria contributed with a study on the assessment of rock-fall hazards. As the Old Town of Salzburg is a prime example of such hazards, systematic GPR-measurements were made on major rock surfaces. The aim of the exploration with GPR was to assess the potential of the method for monitoring rock walls. That is, whether structures in the rock mass which could become important when considering the risk of a rock fall, can be imaged. To manage this problem successfully, moving the antenna over the rough rock surface was made as regular as possible. The effects of vegetation lifting the antenna from the rock surface and of overhanging regions could not be avoided. Due to the loss of coupling of the antenna and the change of radiation pattern, some interference of the records occurred. Thus, only after processing could it become clear which reflections came from within the rock. A three-dimensional survey of discontinuities in the rock requires accurate mapping of the rock surface by photogrammetric methods, to get a digital terrain model for the analysis of the GPR records. The results of repeated measurements have proved to be of essential relevance. They have shown that it is possible to reveal changes with time. For the interpretation of such observations, modeling is a very useful tool.